The present invention relates to a device for sensing the partial pressure of oxygen in a gas, and more particularly to an active multilayer sensor utilizing an oxygen ion conducting material.
It is widely recognized that one of the most important diagnostics for monitoring the efficiency of any combustion process is the measurement of the oxygen partial pressure in an exhaust gas. Thus, oxygen sensors have long been used to measure the oxygen content of exhaust gases from such diverse combustion processes as internal combustion engines in motor vehicles and coal, natural gas, or oil burning power generation facilities.
The most widely known and used oxygen sensors are based on partially stabilized zirconia (PSZ) as the ion conductor. Such sensors function by monitoring the electromotive force (EMF) developed across an ion conductor which is exposed to different partial pressures of oxygen. Oxygen tends to move from a gas containing a high concentration of oxygen to one of lower concentration. If two gases are separated from each other by an oxygen ion conductor, the oxygen molecules will dissociate on one surface of the conductor and absorb electrons to form oxygen ions. These ions then diffuse through the ionic conductor, leaving the entry surface with a deficiency of electrons (O2+4exe2x86x922Oxe2x88x922). On the exit or low oxygen concentration side of the conductor, oxygen ions leaving the conductor must give up electrons to form molecular oxygen, thus leaving the exit surface with an excess of electrons.
This creates the EMF between the two surfaces of the ion conductor. This EMF is described by the Nernst relation:   EMF  =                    t        i            ⁢              (                  RT                      n            ⁢                          xe2x80x83                        ⁢            F                          )              ⁢          xe2x80x83        ⁢    ln    ⁢          xe2x80x83        ⁢          (                        P                      O            2                                    P                      O            2                    xe2x80x2                    )      
where ti is the ionic transference number, R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the electrode reaction (in this case, n=4), F is the Faraday constant, and PO2 and Pxe2x80x2O2 are the oxygen partial pressures of the first and second gases, respectively.
One problem with the use of partially stabilized zirconia sensors is that they must be operated at temperatures in the range of about 800xc2x0 C. to reduce internal resistance to a point where a current can be measured. Further, the raw material costs of stabilized zirconia is relatively high, and the melting point of zirconia is quite high (2700xc2x0 C.) so that formation of sensors is expensive.
Lawless, in U.S. Pat. No. 4,462,891, describes a passive oxygen sensor using ceramic ion conducting materials based on nickel niobates and Bismuth oxides. The oxygen sensor includes a plurality of layers of the ceramic material and a porous metallic conductor arranged to form a body having alternating ceramic and metallic layers, with first alternate ones of the metallic layers being exposed along one side of the body and second alternate ones of the metallic layers being exposed along an opposite side of the body. The first and second alternate ones of the metallic layers are exposed to separate gases, one of the gases being a reference gas, in order to create a voltage output signal across electrodes connected to alternate metallic layers. The voltage output signal is indicative of the relative oxygen partial pressures of the separate gases. Thus, the passive oxygen sensor cannot provide an oxygen partial pressure indication unless the first and second metallic layers present in the body are exposed, respectively, to a sample gas and a separate reference gas having a known oxygen partial pressure, i.e., each side of the sensor body must be exposed to a separate gas.
More recently, amperometric sensors have been introduced which also use partially stabilized zirconia but which do not require a reference gas to operate. Such a sensor 80 is illustrated in FIG. 1 and comprises a cavity 100 in communication with the unknown gas through a diffusion hole 120. The base of the cavity 100 is a PSZ electrolyte 140 which is connected through electrodes 160, 160xe2x80x2 to a voltage source 170. The application of a voltage causes oxygen to be pumped from the cavity through diffusion into the surrounding gas as shown by the arrows. If the cavity is sealed atop the base, and if the top of the cavity has the small diffusion hole 120, then a point is reached on increasing the voltage where no more oxygen can be pumped out of the cavity than is entering through the diffusion hole. The current drawn at this point is called the amperometric current. The larger the oxygen partial pressure in the surrounding gas, the larger will be the amperometric current. Thus, a measurement of the amperometric current yields the oxygen partial pressure. Again, however, this sensor suffers from some of the same drawbacks in that materials and fabrication costs are relatively high. An extremely small diffusion hole is required, about 5 xcexcm, and requires precise machining because the size is critical to the operation of the sensor. Additionally, the manufacture of the sensor of FIG. 1 requires five silk screen operations and four burnout steps. Finally, these sensors lose their sensitivity above about 80% oxygen and the diffusion hole is prone to plugging.
Accordingly, there remains a need in the art for an amperometric oxygen sensor which is relatively inexpensive to manufacture and provides enhanced oxygen sensitivity.
The present invention meets that need by providing a low cost amperometric oxygen sensor which utilizes a plurality of oxygen ion conductor layers interposed between a plurality of oxygen-porous electrode layers. Oxygen from a sample gas enters the sensor at porous cathode electrodes, is pumped through the ion conductor layers, and exits through the anode electrodes. The amperometric current generated is representative of the partial pressure of oxygen in the sample gas.
In accordance with one embodiment of the present invention, an amperometric oxygen sensor is provided for determining the oxygen partial pressure of a gas. The sensor comprises a sensor body defined by a plurality of oxygen-porous electrode layers and at least one oxygen ion conductor layer. The plurality of oxygen-porous electrode layers include at least one cathode layer and at least one anode layer. Each of the cathode layers define first and second major cathode surfaces and each of the anode layers defining first and second major anode surfaces. The oxygen ion conductor layer is interposed between the first major cathode surface and the first major anode surface. The cathode layer defines an unexposed second major cathode surface and a cathode end portion exposed along a first edge of the sensor body. The anode layer defines an unexposed second major anode surface and an anode end portion exposed along a second edge of the sensor body. The amperometric oxygen sensor further comprises a voltage source having a first pole connected to the cathode layer and a second pole connected to the anode layer, and a current meter connected to measure an amperometric current flowing through the at least one ion conductor layer.
The oxygen porous electrode layers define an oxygen diffusion limit that is a function of electrode porosity and oxygen partial pressure of the gas. The oxygen ion conductor layer and the oxygen pump potential define an oxygen pump rate. The oxygen pump rate may be greater than the oxygen diffusion limit.
Preferably, a plurality of cathode layers are provided and electrically connected along the first edge of the sensor body with a first oxygen-porous termination. Similarly, a plurality of anode layers are preferably provided and electrically connected along the second edge of the sensor body with a second oxygen-porous termination.
The sensor body preferably includes a heating circuit. The heating circuit may include a controller programmed to control the resistance of heating electrodes associated with the sensor body by applying a constant current to the heating electrodes and controlling the voltage applied to the heating electrodes. The controller may be further programmed to modulate the pulse width of the constant current to control the heating power applied to the heating electrodes and maintain a constant sensor temperature. The heating circuit comprises at least one heater electrode arranged in a co-planar relationship with at least one of the porous electrode layers. The heater electrode preferably comprises a pair of co-planar heater electrodes separated by a porous electrode material and electrically connected by an interconnect electrode. The interconnect electrode may be arranged on a dielectric cover plate. The interconnect electrode preferably comprises a non-porous electrode and the heater electrode preferably comprises an oxygen-porous electrode.
The oxygen ion conductor layer may comprise a ceramic electrolyte and the oxygen ion conductor layer may comprise partially stabilized zirconia. The oxygen-porous electrode layers may comprise oxygen-porous platinum and may be stabilized against sintering.
The gas may comprise nitrous oxide and the oxygen-porous electrode layers may comprise porous rhodium configured to catalyze dissociation of nitrous oxide into N2 and O2. In this manner, the amperometric current relates to the nitrous oxide content of the gas. For the purposes of defining and describing the present invention it is noted that the term xe2x80x9ccomprisexe2x80x9d is utilized herein in the non-exclusive sense and that a gas, for example, comprising a particular substance may also comprise additional substances.
In accordance with another embodiment of the present invention, a method of determining the partial pressure of oxygen in a gas is provided comprising the steps of: exposing an amperometric oxygen sensor to a gas whose partial pressure is to be determined; connecting a first pole of a voltage source to the at least one cathode layer and a second pole of the voltage source to the at least one anode layer; and measuring an amperometric current flowing through the plurality of ion conductor layers.
Preferably, the sensor body is heated to approximately 550-800xc2x0 C. and the sensor body includes a heating circuit associated with the sensor body. The method may further comprise the step of controlling a resistance of heating electrodes associated with the sensor body by applying a constant current to the heating electrodes and controlling the voltage applied to the heating electrodes. The pulse width of the current may be modulated to control the heating power applied to the heating electrodes and maintain a constant sensor temperature.
According to yet another embodiment of the present invention, a method of producing an amperometric oxygen sensor is provided. The method comprises the steps of: providing an unsintered sensor body, the unsintered sensor body being defined by a plurality of oxygen-porous electrode layers interposed between respective oxygen ion conductor layers; selecting a target porosity for the oxygen-porous electrode layers; selecting a sintering temperature for the sensor body, wherein the sintering temperature is selected to correspond to the target porosity for the oxygen-porous electrode layers; and sintering the sensor body at the selected sintering temperature to yield a sintered sensor body including oxygen porous electrode layers having the target porosity.
Accordingly, the present invention provides a low cost amperometric oxygen sensor which is easy to construct and provides enhanced oxygen sensitivity through amperometric measurement of oxygen partial pressures in a multilayer ceramic capacitor structure.